The current invention relates to the field of towed earth grading or earth scraping apparatus.
Multi-bladed passive earth grading apparatus have been known in the art. Typical of these is the unpatented tractor towed twin blade earth grader commercially sold as a terra-plane sold in the southern United States during the early 1960's. The terra-plane consisted of two blades rigidly mounted between side plates. The entire array being towed by a tractor for earth scraping purposes.
A more elaborate version showing hydraulic actuating means for rotating at least one blade differentially with respect to the second blade is shown in Seal, U.S. Pat. No. 4,320,988. Seal teaches a technique involving the hydraulic rotation or twisting laterally of a spreader blade so as to control the effectiveness of the cut.
The initial multi-bladed passive earth grader mechanisms were ineffective inasmuch as they were suited for only one general type of terrain from their construction and when used on different media, that is when transferred for example, sand to compacted earth, to shell or coarse aggregates, they would become ineffective in efficiently creating a smooth level surface.
The attempt to correct this problem as shown in Seal by differentially twisting the blades produced an extremely highly stressed structure which is prone to breakage and in fact, the Seal mechanism, as patented, is not believed to be in commercial use due to its unsatisfactory performance.